Short wave system



June24, 1941. B. TREVOR I 2,247,212

SHORT WAVE SYSTEM Filed Feb. 19, 1938 2 Sheets-Sheet 1 HIIIHI m &

lll/l/ INV EN TOR.

fiRg/W r/eevoR BY ATTORNEY.

June 24, 1941.

B. TREVQR SHORT WAVE SYSTEM Filed Feb. 19, 1938 2 Sheets-Sheet 2 INVENTOR. BE

RAM TREVOR ATTORNEY.

Patented June 24, 1941 UNITED SHORT WAVE SYSTEM Bertram Trevor, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 19, 1938, Serial No. 191,425

2 Claims.

This invention relates to radio signalling systems having means for rendering them operative over a wide range of frequencies, and particularly to systems for passing a wide band of ultra high frequencies suitable for the transmission or reception of television signals. An important aspect of the invention relates to the tuning circuits of such ultra high frequency signalling systems.

As is known, the radio frequency amplifier customarily used in the art is tuned by means of a circuit employing a variable capacitor associated with a fixed inductor, usually in parallel relation therewith. A tuned circuit of this type is suitable for the passage of a narrow band of frequencies, but has serious disadvantages when applied to wide frequency band systems of the type used for passing wide band television signals. Although it is known that the relatively narrow resonance curve of the usual tuned circuit may be broadened to cover a wider band of frequencies, by introducing series resistance to provide adequate damping of such a tuned circuit, the difficulty with such an arrangement is that the tuned impedance over a wide tuning range varies as the capacitor is varied to tune the circuit, with a consequent variation in gain. Moreover, with such known tuned circuits, assuming a fixed band width, the tuned impedance that may be built up is greatly reduced with increasing values of tuning capacitance. Since the gain per stage is directly proportional to the tuned impedance, it is a desideratum that the tuned impedance be a maximum at all times over the tuning range, particularly when using ultra high frequencies for television signals.

One of the objects of the present invention is to provide an improved wide band radio frequency amplifier suitable for the transmission or reception of television signals.

Another object is to provide an improved radio frequency amplifier which will give uniform and maximum possible gain over the whole tuning range, while maintaining a substantially flat response over the desired pass band.

A further object is to provide an improved unicontrolled wide band multi-stage radio frequency amplifier suitable either for the transmission or reception of television signals.

A still further object is to provide an ime proved tuned circuit wherein the capacitance is maintained at a minimum, in order to obtain a wide band pass with substantially uniform and maximum tuning impedance over the tuning range.

In brief, the present invention utilizes a system wherein the capacitive reactance of the tuned circuit is kept constant at a minimum value, and the inductive reactance varied for tuning purposes. A shunt damping resistor, whose value is low compared with the undamped tuned circuit impedance over the tuning range, when connected across the inductor will give a circuit whose impedance is essentially determined by the resistor, and is therefore constant over the tuning range.

This is shown more clearly if we define the band width b as the frequency difference between two frequencies on the resonance curve where the response is down an arbitrary amount from resonance, say one db (decibel). Tuned circuit theory then shows the percent band width to be approximately proportional to the percent power factor of the circuit. This relation is:

where b is the band width as defined above in cycles, 1 the frequency of resonance, r the equivalent series resistance, a: the inductive or capacitive reactance, and K the constant of proportionality. The above expression can be written i 2 b K r (2) Multiplying both sides of the equation by :1: gives E if -b K r (3) But,

is equal to the tuned impedance which in this case is approximately equal to the shunt damping resistance R.

.Jf-LL on and since 1 7 2m we obtain K 27rRC (6) Equation (6) states that with a constant tuning capacitance c, and a fixed shunt damping resistor R, whose value is low compared with the undamped tuned circuit impedance, a constant band width will be obtained independent of frequency. Since the impedance of the circuit is determined by R, this impedance remains constant throughout the tuning range. Inspection of Equation (6) shows that the band width may be increased by reducing either It or c, and that for a given band width the maximum impedance of the circuit will be obtained with the smallest possible value of c.

In the system of the invention, the maximum impedance obtainable is had at all frequencies over the tuning range and a constant band Width is obtained, since the tuning capacitance c is kept constant at a minimum value, which gives the best possible condition for obtaining maximum gain per vacuum tube stage, since the gain is largely determined by the load presented to the anode circuit of the amplifying tubes.

According to one embodiment of the present invention, successfully employed in practice, the tuned circuits do not require the customary lumped capacitor elements in shunt with the inductors, but, instead, rely upon the Vacuum tube capacity and the lead capacities to the shielded container, as well as the distributed capacity of the inductor coils, to provide the desired capacitance in shunt with the inductors. A.- though in many cases separate lumped capacitors for the tuned circuits are not required, it will be understood, that, if desired, small trimming condensers may be supplied in shunt to the tuning inductor coils of the invention for obtaining identical overall shunt capacity for each of the radio frequency stages.

A feature of the present invention comprises a tuned circuit employing an inductance coil around which there is provided a metal cylinder movable over the length of the coil for varying the effective inductance thereof. This cylinder may or may not make actual "contact with the coil.

A further feature resides in the use of a split gear arrangement for obviating the creation of electrical noise during tuning operations.

Another feature lies in the means employed for supporting each stage and for simultaneously shielding the input and output tuned circuits of each stage from each other. and from adjacent tuned circuits.

The following is a complete description of the invention, given in conjunction with drawings, wherein:

Fig. 1 illustrates, schematically and in crosssection, the essential elements of a tuned radio frequency television receiver embodying the principles of the invention;

Fig. 2 is a perspective View of the back of the receiver of Fig. 1, showing the unicontrol feature, including the gears employed for varying the effective value of the inductors of the tuned circuits. Since the inductors are located in the front of the receiver, they are not shown in Fig. 2. For the sake of simplicity, the wiring between stages has also been omitted from this figure;

Fig. 3 is a front View of one of the split gear assemblies employed for eliminating electrical noise during tuning operations; and

Fig. 4 is a cross-sectional view of the split gear assembly of Fig. 3, along the lines 44.

In the drawings, the same elements are represented by the same reference numerals.

.cuits.

Although the invention is herein described with particular reference to a receiver, it is to be distinctly understood that the principles of the invention are equally applicable to transmitter circuits.

Referring to Fig. 1 in more detail, there is shown a multistage wide frequency band receiving system comprising two vacuum tube radio frequency amplifiers 60, 60 and a first vacuum tube detector 6| with their associated tuned cir- The tuned circuits comprise variable inductance coils 2, 4 and 6. The vacuum tubes are small, preferably of the Acorn type, and are supported in metallic shielding cans 54, 54, which serve not only to support the tubes but also to shield (i. e., isolate) the tuned circuits from one another. These cans are mounted on the chassis 315, in turn maintained at ground or relatively fixed or zero alternating current potential. Each can has a removable cover plate 55.

A suitable source of energy, such as an antenna, is adapted to be connected to the teroperations.

minals 58 of an input or antenna coil 55 which is wound around the free end of and inductively coupled to inductance coil 2.

Coils 2, 4 and 6 are equal inductances space wound with heavy wire on a threaded insulating form. Coils 2 and 6 are wound with a left hand thread and coil 4 is wound with a right hand thread. The coils 2,4, 6 are supported by metal studs 8, I0, I 2, which studs are threaded to the same pitch as the coils. Metal or conducting cylinders I I, I6, I8 are arranged to have clearance between the inner surfaces of their walls and the coils, but are tapped at their lower ends in order to screw smoothly on the studs 8, I0, I2. Gear wheels 20, 22, 24 are clamped by rings 2I, 23, 25 to the base of their respective cylinders I4, I6, I8. These clamps 2i, 23, 25 may be loosened, thus freeing the gears from their respective cylinders, to allow individual adjustment of any one of the inductances by turning a particular cylinder without altering the others. The train of gears 22, 22, 24 is driven in unicontrol fashion by the large gear 26, which may be turned by the operator by means of crank or dial 28, in turn linked to gear 26 by means of shaft 21.

At the upper end of each metallic cylinder I4, I6, I8 are fitted spring contacts 38, 32, 34

which press against the bare wire of coils 2, 4, 61

One or more spring contacts may be used at the end of each cylinder to make contact to'the coils. The coilsand contacts are arranged so that equal inductances of coils 2, 2, 6 are had at any position of the cylinders I4, I6, I8 with respect to the coils. The drum 26 and gears 20, 22 and 24 thus provide 'unicontrol action for the coils.

In order to insure good contact between the cylinders I 5, I 6, t8 and the grounded receiver chassis 36, there are supplied spring contacts 38, 46, 42 which provide a radio frequency path to ground for each cylinder, thus preventing electrical noise from being generated during tuning A further aid in this respect is obtained by using split gears for 20 and 24 which are provided with anti-backlash springs 9, 9. There aresix springs for each split gear, three on each side. (Note Figs. 2, 3 and 4.) The antibacklash feature maintains good electrical contact between the gears and eliminates electrical noise by keeping any circulating radio'frequency current in the gear train from changing due to variable contacts. between the gears.

Mechanical stops it and 48 are provided at the ends of the tuning range. Studs 46 and 48 may be screwed in and out as desired in order that pins 44, adapted to engage said studs at opposite ends of the path of travel of said studs, will engage with them at the desired locations.

An indicating drum 52, which may be calibrated to show the mid-band frequency, is driven by the worm wheel 55 which meshes with the worm gear cut in the shaft 2'! of the main drive gear 26.

Figs. 2, 3 and 4 show, among other things, the details of the gearing assemblages.

It should be noted that split gears 20 and 24 each comprise two outer gears 3 and I and a central gear 5. Each outer gear is fastened to the central gear 5 by means of three anti-backlash springs 9, 9, the ends of these springs being fastened to pins I I, I i located on both the inner and outer gears, as shown. The six springs fastened to both sides of the central gear of each of the split gear assemblages 2B and 24 are designed to pull in the same direction, let us say, in the direction of the arrows.

In order to tune the receiving system, the crank or dial 28 is turned, in turn rotating gears 26, 20, 22 and 24. The movement of gears 20, 22 and 24 will cause cylinders I4, I6 and I8 to move simultaneously and to the same extent over the lengths of their respective inductors 2, 4 and 6, thus varying, equally, the effective inductances of the coils.

If desired, damping resistors may be connected across the inductance coils 2, 4 and 6 to give the desired tuning band width for the system. It should be noted that the only capacitance used in the tuned circuits having coils 2, 4 and 6 is that provided by the vacuum tube capacities and the capacities of the high potential leads to the chassis, as well as the distributed capacity of the inductor coils 2, 4 and 6. The capacitance of each tuned circuit is thus kept at a minimum, consistent with circuit limitations, and of constant value. It will be appreciated that the smaller the value of the capacitance, the greater the tuned impedance can be built up for a given band pass. Maximum gain is thus obtained consistent with the band width used. This gain is constant over the entire tuning range of each tuned circuit. If desired, the system may be operated without the use of damping resistors, in which case the gain and band width would be determined by the power factor of the coils and the associated circuits. Where damping resistors are not used, a more accurate trimming of the tuning capacitances would be required, and it would in such case be preferred to add a small trimming capacitor in shunt with each tuning inductor 2, 4 and 6.

It should be understood, of course, that the invention is not limited to the precise arrangements of parts shown, since various modifications may be made without departing from the spirit and scope thereof. For example, the spring contacts 3U, 32, 34 and 38, 40 and 42 of Fig. 1 may be replaced by rolling contactors. Also, the studs 8, I 0 and I2 can be insulated from the chassis 36, and by-pass capacitors supplied to provide a low impedance radio frequency path from those studs to the chassis. This would allow the inductor coils to carry the vacuum tube anode currents, with some slight modification of the circuit connections. Also, the use of gears composed of insulating material, such as Bakelite, might be used to advantage in place of the metal gears 20, 22, 24 for the purpose of eliminating circulating radio frequency currents between the gears. This construction would allow elimination of the split gear arrangement shown at 23 and 24 of Fig. 2.

What is claimed is:

1. An ultra high frequency radio signalling system operative over a wide range of frequencies including a plurality of radio frequency tuning circuits, each tuning circuit having a variable inductive reactance and being devoid of concentrated shunt tuning capacitance, whereby there is obtained minimum capacitive reactance for said tuning circuits and uniform and maximum tuning impedance and uniform width of resonance curve over the tuning range, each of said inductive reactances comprising a coil of Wire mounted on an insulating form and whose individual turns are spaced from one another, and a metallic enclosure adjacent to and movable over the greater portion of the length of said reactance for varying the effective value thereof; a shielding can individual to and surrounding that portion of each inductive reactance not covered by said movable metallic enclosure, and unicontrol means for simultaneously moving, equally, all of said metallic enclosures over their associated reactances in the same direction, whereby the effective values of said inductive reactances are varied to the same extent, said unicontrol means including a gear assemblage for each tuning circuit, the gear assemblage for certain tuning circuits being constituted by a central gear disc having on both sides thereof additional gear discs linked thereto by antibacklash springs, the gears of said tuning circuits being intermeshed and linked to a control shaft.

2. An ultra high frequency radio signalling system operative over a wide range of frequencies including three radio frequency tuning circuits, each tuning circuit having a variable inductive reactance and being devoid of concentrated shunt tuning capacitance, whereby there is obtained minimum capacitive reactance for said tuning circuits and uniform and maximum tuning impedance and uniform width of resonance curve over the tuning range, each of said inductive reactances comprising a coil of wire whose individual turns are spaced from one another and wound on a threaded insulating form, said forms being positioned in a row parallel to one another, the center coil being wound oppositely relative to the outside coils, and a metallic enclosure adjacent to and movable over the greater portion of the length of said reactance for varying the effective value thereof; a shielding can individual to and surrounding that portion of each inductive reactance not covered by said movable metallic enclosure, and unicontrol means for simultaneously moving, equally, all of said metallic enclosures over their associated reactances in the same direction, whereby the effective values of said inductive reactances are varied to the same extent, said unicontrol means including a gear assemblage for each tuning circuit, the gear assemblage for the outer tuned circuits being constituted by a central gear disc having on both sides thereof additional gear discs linked thereto by anti-backlash springs, the gears of said tuning circuits being intermeshed and linked to a control shaft, and studs for limiting the range of axial movement of said gear assemblages.

BERTRAM TREVOR. 

